To ease the process of drilling into a formation and to improve production yields for desirable formation fluids, the oilfield-services industry has developed processes and tools for measuring the properties of the formation during drilling or shortly thereafter, a process called “logging while drilling” (LWD). Integrating logging with drilling not only shortens the time needed to make the well operational, but also allows the drill operator to measure the formation's properties before drilling fluid invades the formation and obscures its properties. LWD tools and methods evaluate multiple parameters, such as formation resistivity, porosity, and various acoustic characteristics, to determine the types of minerals and/or fluids that comprise the formation. Generally, logging tools and processes involve radiation of energy, such as acoustic or electromagnetic waves, into the formation to induce energy from the formation. The characteristics of the induced energy reveals properties of the formation. Properties of particular value in the drilling process are the identity, location and relative proportions of minerals and fluids present in the formation.
LWD tools incorporate measurement devices into the bottomhole assembly in order to obtain measurements as soon as possible after the well wall is drilled. LWD tools have thus been attached to the drill string, for example, in specially adapted drill collars located just above the drill bit. The sensors incorporated into these LWD tools will typically detect formation properties at a given well depth just after the drill bit drills past that depth. While positioning the LWD tools just above the drill bit yields information about the formation before extensive drilling fluid invasion can occur, drilling fluid invasion can still mask properties of the formation.
While the present invention is susceptible to various modifications and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific examples is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and the scope of the invention as defined by the appended claims.